We've moved please click to go to our new siteWWW.BISQUEWARE.CO.UK
|
|
|
GLOSSARY
OF POTTERY AND CERAMICS TERMS
A
wash of refractory material painted onto kiln furniture to protect against glaze
runs. Biscuit/Bisque The
first firing, converting clay to ceramic which cannot be reconstituted in water.
Bisque is used to describe a
first firing that is higher than the subsequent glaze firing. Biscuit
describes the reverse where the glaze firing is the higher, also the ware
produced by these firings. Bloating Lumps
on the surface of a fired piece caused by expansion of gasses within the piece
generated by overfiring. Blunger Machine
for mixing clay and water in the production of casting slips Body A
manufactured clay, where raw clays and minerals are blended to produce specific
qualities. Calcination/Calcining Heating
to purify and prepare materials for use. Ceramic Clay
that has been altered by firing to produce a hard substance that cannot be
reconstituted in water. The general
name given to all fired clayware. Coiling Making
pots by buiding up coils of clay Cones Small
pyramids of specially prepared clays formulated to bend at specific temperatures
giving accurate measurment of heatwork generated
in the firing. Crazing A
network of small cracks in the glaze surface caused by a poor glaze fit. The glaze
contracts more than the clay and the resulting tension causes it to crack.
If a glaze contracts after firing more than the clay, the glaze being
unable to stretch, starts to crack under the tension.
The glaze will continue to crack until the tension is eased as crazing
covers the surface and becomes finer. Delayed
crazing can occur days or even months after firing.
The most common cause of crazing is underfired biscuit.
If the clay does not reach its full maturing temperature either in the
bisque or glaze firing, it will not contract sufficiently to fit the glaze.
Crawling A
glaze fault where the glaze has rolled back in the firing leaving a bare patch,
caused by dirt, dust or grease on the pot before glazing and by the glaze being
stiff, or applied too thickly and not flowing easily in the firing. Decal A
transfer or lithograph used to apply repeat decoration, usually on-glaze. The
decoration is printed in ceramic colours on to a backing which is removed and
the design transferred to the glazed ware, which is then re-fired to a lower on-glaze
temperature. Deflocculation Separating
and dispersing the clay particles in a slip making it more fluid. Sodium
Carbonate and Sodium Silicate are deflocculants, when disolved in the slip they
allow large proportions of clay to remain fluid giving a high density to the
slip. Dryfooting Leaving
the footring of a pot unglazed so that it can be fired standing on the kiln
shelf without being stilted. Dunting Cracking
of pots due to stress built up by Silica
Inversion in the firing. Rapid
and uneven heating and cooling of the pot in firing is the most common cause,
pots made from uneven thicknesses of clay are also at risk and poor glaze
fit can also be a cause Earthenware Porous
ware, lower fired ware usually under 1200°C and the clay and glazes that it is
made from. Earthenware is usually characterised by brighter colours, softer
glazes and separate glaze layer over the body. Element A
high temperature resistance wire, wound in a coil, which carries electrical
current for heating a kiln. Enamel Coloured
glasses used to decorate metal and ceramics, see On-glaze. Engobe Usually
used to mean decorating slip. A
covering over the base clay not firing to a glassy finish. Eutectic
Mixture The
mixture of substances in proportions that produce the lowest possible melting
point given the fluxing action in combinations of materials.
The eutectic mixture of
Alumina and Silica for instance is 10% Alumina and 90% Silica which gives a
melting point of 1545°C. This is
lower than the melting point of Alumina at 2050°C or Silica 1710°C. Fettling Cleaning
of cast ware, especially the removal of the seams produced by joins in the
plaster mould. Flocculation The
opposite of deflocculation, the
thickening of slips or slops
to give a bulkier and more viscous liquid.
In flocculation the particles collect together rather than repel each
other as in deflocculation.
Calcium Chloride and vinegar are commonly used flocculants. Fluidity The
liquidity of slips and molten glazes. Used to describe free flowing slips and
glazes which melt to a runny consistency in the firing. The opposite of viscosity. Flux Substances
that encourage fusion in the firing to produce ceramic. An essential
ingredient in both clays and glazes, fluxes
interact with Silica glass formers producing the solid, fused ceramic
of the pot and fused glass of the glaze. Some
of the commonest fluxes are: Boric Oxide, Calcia, Lead Oxide, Potash, Soda. Frit A
fusion of soluble or harmful glaze materials in a glass that renders them
insoluble and safe so that they can easily be incorporated in the glaze. Glaze The
glassy layer covering the surface of pots to provide a non-porous, decorative
and usually smooth surface giving increased strength to the pot. Glaze Fit The
matching of the thermal expansion and contraction of the glaze to the clay body.
If glaze and body
contract at different rates the faults of crazing,
shivering or dunting can occur. Glost Glaze
firing, especially where the glaze
firing is lower than the bisque Greenware Unfired
clayware Grog Fired
ceramic ground to specific sizes.
Added to clay bodies to give openess to aid drying without warping,
'tooth' to aid forming and to reduce shrinkage Hardening-on The
firing of underglaze colours before
applying glaze to prevent crawling,
glaze smudging of the colour and contamination of the glaze and to aid glaze
application. Heatwork Temperature
x Time. To produce ceramic change
in firing it is necessary not only to reach certain temperatures but to expose
the ware to temperature for a period of time. Heatwork
is measured by Pyrometric cones, Bullers rings and Minibars.
In-Glaze Decoration
of colour or oxide applied to the unfired glaze
surface. The decoration is
therefore neither underglaze nor on-glaze
but in-glaze. Majolica
and Delft ar examples of in-glaze
ware. Jigger/Jolley Clay
forming machine. The Jigger rotates
clay over a mould which forms the inside shape of the pot, the outside shape is
produced by a former being pressed against the clay as it revolves.
The Jolley is th reverse, it
forms clay by rotating it while being held in a mould which shapes the outside
of the pot, a former in the profile of the required design is forced into the
clay creating the inside shape. Kiln Sitter A
mechanical device for shutting off kilns when the firing is complete. Kiln
Sitters are activated by the bending of Minibars
when they reach their melting point. Kneading The
manual working of clay to produce a homogenous smooth and plastic mass ready for
throwing, hand building or modelling. Kneading
follows wedging in clay
preparation and is often referred to as wedging. Lawn The
fine wire mesh used for sieving materials.
Usually made from phosphor-bronze and measured by the number of apertures
per linear inch. A 60's mesh lawn
would have 60 apertures per inch. Low Sol. Low Solubility. The name given to glazes incorporating metals, especially
lead, which are present in a form that is not readily dissolved by the action of
acid on the fired ware. Low Sol. glazes conform to Health and Safety requirements and are
therefore safe for dinnerware when fired correctly. Lustre Precious
metals in liquid suspensions that are applied to glaze fired ware and fired to low on-glaze temperatures to produce a metalic surface. Majolica Painted
decoration of oxides or decorating colour applied to a white tin glaze before
the glaze is fired. See
In-glaze. Maturity The
temperature, or heatwork, range at
which Clays reach their optimum strength and glazes achieve the desired surface quality and colour. Minibars A
form of Pyrometric cone but in a bar
shape specifically designed for use with
Kiln Sitters. Opacifier Material
for making transparent glazes opaque or white. Tin and Zircon are two common opacifiers. On-Glaze Colour
decoration applied to fired glaze and fired again to between 680°C and 880°C Once-Fired Ware
that has been fired once, i.e. glaze
applied to greenware with glaze
and body maturing together. Oxidising Firing
in a kiln atmosphere where oxygen is present. Electric kiln firings are always
considered to be in oxidising
atmospheres. Peeling See
shivering Pin Holes Small
holes in the glaze surface resulting
from bubbles of gas coming through the glaze
in its molten state and leaving craters that have had sufficent time to heal
over. Causes are; poorly deflocculated
casting slips, underfired biscuit and
not long enough firings. Pint Weight A
measure of density used to enable slips or glazes of matching density to be
reproduced. It is the weight of a
pint of the liquid expressed in ounces e.g. 30 oz pt. The metric equivalent is
gramms per 100ml. 30oz pt. equals
150 g/100ml. Porosity The
quality of fired ware to absorb water. Lower fired ware is generally more porous
and absorbs water readily while ware fired to vitrification will have practically no porosity. The correct level
of porosity is important in glazing
where the rate of water absorbtion afects the thickness of the glaze
layer. Pugmill A
machine for processing and reconstituting clay. Scraps of used clay are
compressed by an auger through a tapering barrel and extruded. A De-airing pugmill
has the same function with the addition of a vacuum pump to remove greater
amounts of air. Pyrometer A
temperature measuring instrument. Pyrometers
measure kiln temperatures by interpreting the micro-volt output of a thermocouple.
Raku A
method of firing where pots are removed from the kiln at maturing temperature. After removing from the kiln Raku
pots are often enclosed in sawdust or other combustible material to create a reducing
atmosphere where bright metallic colours can often be produced. Raw Glaze A
glaze made of un-fritted materials. Raw Glazing Once-firing. Reduction Firing
in an atmosphere where the oxygen content is reduced and oxygen is removed from
metal oxides creating different colours in clay and glaze. The immersion of
Raku pots in saw dust is described as post-firing reduction. Refractory Substances
that are resistant to high temperatures such as kiln furniture. Salt
Glazing Glazing
with common salt which is thrown into the kiln at temperatures above 1100°C.
The salt turns to vapour and the Soda content combines with Alumina and Silica
in the clay to produce a glaze. Sgraffito Decorating
technique involving scratching through a slip
or glaze layer. Shelling See
shivering. Shivering A
glaze fault occuring when the body
contracts at a greater rate than the glaze,
putting the glaze under compression
and causing it to separate from the body
and flake and peel off, particularly on edges of potts. Shivering is the opposite fault to crazing. Silica
Inversion The
changing of the structure of Silica molecules at particular temperatures or
Inversion Points, which causes expansion above the point or contraction below
it. It is this change of size that creates the stresses that result in dunting.
The inversion point of Silica in the Quartz phase is between 550°C and 575°C
but is generally taken to be 573°C. In
the Cristobalite phase it is between 220°C and 280°C and is taken to be 225°C. Sintering The
sticking together of materials prior to melting. Slip Clay
in a liquid form for casting or decorating. Slip
Casting Method
of reproducing shapes in clay by moulding liquid slip in plaster moulds. The
plaster absorbs water from the slip leaving a coating of clay next to the mould
surface taking the shape of the mould. Slop The
name given to a glaze after it has
been mixed with water ready for glazing. Soak Holding
the temperature for a given time during the firing to enable the kiln
temperature to even out and to allow glazes to become smooth while molten
without overfiring. Spit Out Calcium
impurities in a clay body can absorb
moisture over time which causes them to swell and burst through the glaze
layer leaving a crater in the pot. The bottom of the crater will typically have
a small white centre confirming the presence of Calcium. Stoneware Ceramic
ware and the clays and glazes from which it is made. Stoneware is fired high
enough to produce a low porosity body.
Usually taken to be ware fired above 1200°C.
Stoneware is also characterised by the integration of the glaze
and the body.
Sprigging Embossed
decoration on clay ware, usually press
moulded shapes applied to the pot to give a raised decoration. Thermal
shock Sudden
change of temperature in a fired pot that creates stress due to expansion and
contraction. Oven ware must be made of a clay that withstands the thermal
shock of repeated cooking. Thermocouple An
instrument for measuring temperature in the kiln. Thermocouples are
the ceramic probes that protrude into the kiln.
Wires inside the thermocouple generate
a small current which is measured by a Pyrometer
which displays the temperature. Thixotropy The
thickening of liquids when they are left undisturbed. A slip left for a few days will become less viscous as soon as it is stirred. Thixotropy enables the clay in a mould to keep its shape when newly
cast and still wet. Throwing The
forming of round pots by rotating clay on a potters' wheel.
The clay is driven through the potters' hands by the force of the wheel
and the position of the hands determines the shape of the pot. Transfer See
decal. Turning The
trimming off of surplus clay on thrown pots.
This is done at the leather hard
stage and is usually confined to the outside and base of pots. Underglaze Decoration
applied to pots which are subsequently glazed with a transparent glaze.
Underglaze colours are
protected from wear by the glaze layer
on top of them. Venting Allowing
the escape of gasses both from the kiln during firing and the kiln room. It is
important that all kilns and kiln rooms are vented adequately to prevent the build up of harmful gasses. Viscosity The
opposite of fluidity, the stiffness
of a slip or a molten glaze. Viscous slips
pour slowly and viscous glazes move
very little in the firing. Vitrification The
point at which clay becomes virtually non-porous and solid, almost glassy. Vitrification
is the stage before melting when the pot would slump under its own weight. Volatilisation Vaporization.
Some glaze materials volatilise in
the kiln atmosphere becoming gasses which can then be lost through the kiln flue
or they can combine with materials on the surface of other pots giving flashes
of colour. Chrome and Copper are materials prone to volatilisation
and salt glazing relies on this phenomenom. Wax Resist A
decorating technique where liquid wax is used to create areas of pattern that
repel a covering colour or glaze. Wax
resist can be applied in several layers, each layer blocking out more of the
origional colour. The wax is burned away during firing. Wedging The
process that compresses clay removes air bubbles and prepares it for use. A lump
of clay is thrown down on to a work surface, cut in half with a wire and the
half piece thrown down on the remainder. This process is the manual equivalent pugging
and is followed by kneading. HEALTH
AND SAFETY
Pottery is safe
and free from health and safety risks as long as the basic guidelines listed
here are followed. Some pottery
materials are toxic, but then so are many of the products used every day in our
homes. We have all learnt how to
handle bleach, cleaners, paints and so on, we must use some pottery materials
with the same caution. The
following simple rules will eliminate risk from the pottery workplace: 1.
Avoid creating dust. Many
materials not considered toxic when eaten can be regarded as hazardous in dust
form. Silica dust from clays and glazes is the main problem and if
breathed in large amounts over long periods of years can cause silicosis.
This disease has largely been eradicated from the pottery industry and
the same sensible working methods will prevent problems in other environments
where generally the exposure is much lower. 2.
Clean up spills before they dry. 3.
Don't generate dust when brushing, use an industrial standard vacuum
cleaner and damp down floors and work surfaces.
4.
Use wet cleaning processes where you can. 5.
Wear protective clothing and keep the clothing in the workplace to avoid
distributing the dust. See
Cromartie safety equipment section. 6.
Always clean tools and equipment after use. 7.
Don't eat or smoke in the workplace. 8.
Keep the workplace well ventilated.
9.
Keep materials well wrapped and clearly labelled. 10.
Dispose of empty wrapping carefully. 11.
Always use a spray booth when spraying slips or glazes. Metal
Release in Glazes Some glazes use
lead and other metallic compounds which can be released by the action of some
acidic foods and drinks. Standards have been established to ensure that metal
release levels are kept to a safe minimum and our glazes recommended for
dinnerware are within this safe level.
Leadless
and Low Sol Glazes Where lead is
used in glazes it is present only in a "fritted" form where the lead
is combined with silica in glass that renders it safe both in use and when
fired. Glazes of this type that
conform to metal release tests are called Low Lead Solubility or "Low
Sol" glazes and are safe for dinnerware use.
Lead is used for its ability to enhance the brightness of colours and
oxides, leadless glazes may sometimes not achieve the same brightness.
Where no lead is used in the glazes formula glazes are called
"Leadless". Leadless
glazes may still contain traces of lead though only in insignificant amounts. Metal
Release in Glazes Where lead and
other metallic compounds are used in glazes these substances can sometimes be
released by the action of acidic foods and drinks.
Standards have been set to ensure that metal release levels are kept to a
safe minimum and all our glazes recommended for dinnerware conform to these
standards when fired to the recommended temperatures.
While leadless glazes can be modified with additions of glaze materials
without problem, additions made to Low Sol glazes could have the effect of
altering the metal release characteristics.
Small amounts of most glaze materials would not usually cause a problem
but copper and zinc should not be added to Low Sol glazes. Products sold as dinnerware or cookware must conform to metal
release standards and it is the responsibility of the potter to ensure the
safety of the finished article. We
can provide more information on the availability of test facilities if required. Specific
Materials The following
materials are the ones in our range which are designated harmful by inhalation
and ingestion: Barium Carbonate,
Calcium Chloride, Chromium Oxide, Cobalt Carbonate, Cobalt Oxide, Copper
Carbonate, Manganese Dioxide, Nickel Oxide, Vanadium Pentoxide, Colours and
Stains. Brush-On
Colours and Glazes Our
Brush-On colours and glazes in the Duncan and Gare ranges are manufactured
subject to stringent American controls. The
respective catalogues detail all the products as either non-toxic, dinnerware
safe or decorative use only. In the
liquid Brush-On form colours and glazes contain gums and suspending agents which
dry to hard, dust free surfaces and therefore virtually eliminates the dust
problem making them especially safe to use. Kiln
Firing The
hazards of using kilns from the point of view of electricity and heat are
obvious, there are also some other problems detailed below.
1.When
using kilns always ensure that the manufacturers instructions are followed
accurately and no alterations are made to the kiln without manufacturers advice. 2.Remember that the
outside of a kiln can get hot enough to burn especially after a long, high
temperature firing. 3.For
safety, kilns should be sited suitably to prevent unauthorised use or to keep
children away. 4.Avoid
working in a room where a kiln is firing unless it is well ventilated and the
kiln is ducted to atmosphere. Ware
fired in kilns produces potentially harmful vapours in all types of firing and
in particular when firing lustres. Ask
for Advice If
you would like more information or if you have a specific Health and Safety
query, please don't hesitate to call us. If
you are opening a new department or studio we will be pleased to advise on all
aspects.
|
60 Years of trading!Contact Information
e-mail your enquiry to us at enquiries@cromartie.co.uk
|
